1. Field of the Invention
This invention relates to an electric motor, and more particularly to an electric motor suitably applied as a stepping motor which includes a stator having a plurality of stator windings wound on bobbins and a rotor including a permanent magnet magnetized to provide north and south poles alternating on the periphery thereof in a predetermined angular spaced relationship in a circumferential direction around an axis thereof.
2. Description of the Prior Art
A stepping motor having a structure of snap-coupling a stator assembly in an integral relationship without the necessity of welding or some other retaining means is disclosed in U.S. Pat. No. 4,174,485. In accordance with the structure, an electric motor includes a stator having a first cup shaped member having an annular wall which is disposed substantially in a perpendicularly relationship to the axis of the rotor, a circumferential surface portion extending from an outer peripheral edge of the annular wall and disposed in a substantially concentrical relationship with the axis, a plurality of magnetic pole pieces extending from an inner peripheral edge of the annular wall and disposed in a spaced relationship by a predetermined angular distance around the axis, a plurality of tabs extending in the extending direction of the circumferential surface portion from a free end of the circumferential surface portion and a plurality of cooperatively dimensioned and configured recesses, and a second cup shaped member having an annular wall, a circumferential surface portion, a plurality of magnetic pole pieces, a plurality of tabs and recesses which are all configured similarly to those of the first cup shaped member. The width of an end portion of each of the tabs is greater than that at a base portion of each of the tabs at which the tab connects to the free end of the circumferential surface portion of each of the cup shaped members, and the inclination of a side edge of each tab is set to about one degree. Consequently, when the tabs of the first cup shaped member are connected to the recesses of the second cup shaped member and the tabs of the second cup shaped member are connected to the recesses of the first cup shaped member, the first and second cup shaped members are snap-coupled to each other due to interference between the tabs and the cooperating recesses because of the presence of the taper of about one degree. Accordingly, the first and second cup shaped members are formed into a single assembly without the necessity of welding or the use of some other additional fixing means between the first and second cup shaped members.
Each of the first and second cup shaped members has a plurality of additional recesses formed with an angularly spaced relationship therein. The electric motor further includes first and second magnetic pole plates each having a plurality of upright magnetic pole pieces disposed in a substantially parallel relationship to the axis of the rotor on an inner peripheral edge of a planar annular member, and a plurality of tabs formed with a predetermined angularly spaced relationship in a plane of the planar annular member on an outer peripheral edge thereof and dimensioned and configured such that they may fit in the additional recesses formed in the first and second cup shaped members. Thus, priorr to snap-coupling of the first and second cup shaped members to each other, stator windings are installed and secured in annular spacings between the circumferential surface portions of the first and second cup shaped members and the magnetic pole plates, and then the tabs formed in the planes of the planar annular members of the first and second magnetic pole plates on the outer peripheral edge thereof are snugly fitted in the additional recesses of the first and second cup shaped members to assemble the first and second magnetic pole plates to the individual cup shaped members, thereby attaining snap-coupling of the first and second cup shaped members and forming a single stator assembly.
With the structure of the stator assembly, the tabs formed on the first magnetic pole plate are snugly fitted in the additional recesses of the first cup shaped member. Accordingly, the magnetic pole pieces formed on the first cup shaped member and the magnetic pole pieces formed on the first magnetic pole plate are prevented from relative movement in the angular direction around the axis of the rotor due to the snug fitting of the tabs and recesses. Meanwhile, the magnetic pole pieces formed on the second cup shaped member and the magnetic pole pieces formed on the second magnetic pole plate are prevented from relative angular movement around the axis of the rotor due to the snug fitting of the tabs and recesses. Subsequently, the plurality of tabs extending in the extending direction of the circumferential surface portion from the free end of the circumferential surface portion of the first cup shaped member are bound in the recesses formed at the free end of the circumferential surface portion of the second cup shaped member due to mutual interference by the snap-coupling as described above. Meanwhile, the plurality of tabs extending in the extending direction of the circumferential surface portion from the free end of the circumferential surface portion of the second cup shaped member are bound in the recesses formed at the free end of the circumferential surface portion of the first cup shaped member due to mutual interference by the snap-coupling as described above. Accordingly, angular movement of the rows of the magnetic pole pieces formed on the second cup shaped member and the second magnetic pole plate relative to the rows of the magnetic pole pieces formed on the first cup shaped member and the first magnetic pole plate around the axis of the rotor is prevented by the snap-coupling described above. With the structure described above, positioning of the magnetic pole plates is achieved at two stages in this manner.
Meanwhile, snap-coupling in binding the individual tabs of the first and second cup shaped members in the recesses of the second and first cup shaped members by mutual interference utilizing the negative taper of about one degree of the side edges of the tabs is established by pushing the tabs in the direction of the axis of the rotor into the recesses. The binding force at the connecting portions bound by the snap-coupling depends upon the thickness of material of the circumferential surface portions of the cup shaped members on and in which the tabs and the recesses are formed and also the residual distortion stress upon binding by interference.
In a stepping motor which is normally produced, the tabs are apt to be coupled in an outwardly or inwardly displaced condition in a radial direction with respect to recesses because the first and second cup shaped members have a thickness of material of 1 mm or so. Accordingly, there is the possibility that magnetic pole pieces formed on the first and second cup shaped members may be located in an eccentric relationship from each other with respect to the axis of the rotor after coupling of the first and second cup shaped members or may be angularly displaced out of a predetermined angularly equidistantly spaced relationship with respect to the axis of the rotor. After construction of a stepping motor, the eccentricity of the magnetic pole pieces will cause a problem in that the magnetic pole pieces may interfere with the rotor, resulting in incomplete operation of the motor. To the contrary, the angular displacement of the magnetic pole pieces will cause another problem in that the error in stepping angle may increase so as to deteriorate the resulting accuracy. Where a coating of resin material is molded on an outside of the stator structure, those problems are more significant on account of the influence of heat.
Further, since the side edges of the tabs having a negative taper are bound with the side edges of the recesses by mutual interference upon snap-coupling between the first and second cup shaped members, a the extent of press fitting required is great. Accordingly, the binding portions of the cup shaped members are readily scraped so as to cause chips to be formed. Such chips will cause a problem in that, if they enter the inside of the cup shaped members, they may stick to a surface of the rotor due to magnetic force of the rotor so as to cause incomplete operation of the motor.